1. Field of the Invention
This invention relates to a method of nondestructive insulation testing of an electric apparatus, and a nondestructive insulation testing apparatus for nondestructively testing insulation of an electric apparatus.
2. Description of the Prior Art
There are various insulation tests for detecting scratches on surfaces of insulation coats on windings through applying an electric signal. The scratch may be generated by processes such as assembling windings with a laminated core. The scratch may cause a dielectric breakdown during running of an electric apparatus having a motor including its windings.
For example, a surge test applying a surge signal to a laminated core and windings provided to the laminated core at atmospheric pressure is known. This surge test can detect scratches apart from each other by less than 1 mm or a scratch apart from the laminated core by less than 1 mm.
A corona discharge test in the atmospheric pressure can detect scratches having a distance less than 1 mm therebetween or from the laminated core.
A reduction pressure corona discharge insulation test is also known. In this corona discharge insulation test, an ac voltage is supplied to the sample to be tested in a container where a pressure is lower than the atmospheric pressure in the condition that a corona discharge readily occurs in accordance with the Paschen's law to detect a scratch or the like on a surface of an insulation coat of a winding of a coil.
FIG. 7 is an illustration illustrating a prior art method of corona discharge insulation test. A motor coil 161 to be tested is placed in a tank 162. The pressure in the tank 162 is reduced by a vacuum pump 165 through a valve 164. A vacuum gauge 163 detects the pressure.
An ac voltage 168 is applied to the motor coil 161 through a bushing 166 and a laminated core 161a of the motor is coupled to a corona detector 610 through a bushing 167 and a filter 167. The corona detector 610 detects an amount of corona discharge.
FIG. 8 is an illustration of a prior art example of scratches on windings of coils.
Numeral 71 denotes the laminated core and supports coils 72. Scratches 73 and 74 are close to the laminated core 71 and the scratch 73 is close to the scratch 74 by one to two millimeters. Scratches 75 and 76 are about 30 millimeters apart from the laminated core 71 and the scratch 75 is close to the scratch 76.
The scratch 73 can be readily detected by detecting corona discharge in a reduced pressure with 300 to 500 volts. The scratch 74 is detected when a mode of the discharge changes from the corona discharge to the glow discharge by applying a high voltage from 700 to 1400 volts.
However, in such testing where the discharge is provided in a reduced pressure with application of an ac voltage having 60 Hz, an energy of the discharge is large, so that insulation at a peripheral portion of the scratch is destroyed considerably. Therefore, this test cannot be applied to a nondestructive test.